Exostosin family. The EXT family is a family of tumour suppressor genes. Mutations of EXT1 on 8q24.1, EXT2 on 11p11-13, and EXT3 on 19p have been associated with the autosomal dominant disorder known as hereditary multiple exostoses (HME). This is the most common known skeletal dysplasia. The chromosomal locations of other EXT genes suggest association with other forms of neoplasia. EXT1 and EXT2 have both been shown to encode a heparan sulphate polymerase with both D-glucuronyl (GlcA) and N-acetyl-D-glucosaminoglycan (GlcNAC) transferase activities. The nature of the defect in heparan sulphate biosynthesis in HME is unclear.